1. Field of the Invention
The present invention relates to a device for controlling the vehicle to travel at a constant speed by using an automatic transmission having an overdrive position. More particularly, the invention relates a device for controlling the vehicle to travel at a constant speed, which clearly distinguishes a downhill road over a flat road while the vehicle is traveling at a constant speed, and resets or resumes the overdrive position in order to stably maintain a vehicle speed at all times.
2. Prior Art
There has heretofore been known a device for controlling the vehicle to travel at a constant speed by using an automatic transmission having an overdrive position as disclosed in, for example, Japanese Patent Laid-Open No. 59-126142.
The conventional device for controlling the vehicle to travel at a constant speed will be described below.
In the vehicles equipped with an automatic transmission, it is a tendency to mount a constant-speed traveling device for controlling the traveling speed to acquire a preset target vehicle speed.
When the device for controlling the vehicle to travel at a constant speed is used, the vehicle that is normally traveling is so controlled as to travel at the overdrive position at a constant speed by suppressing the engine rotational speed to a minimum limit in order to save fuel.
When the vehicle traveling at the overdrive position at a constant speed now arrives at steep downhill, the effect of the engine brake is so weak that the vehicle speed may become considerably greater than a target vehicle speed despite the throttle valve is fully closed.
According to the device for controlling a vehicle to travel at a constant speed disclosed in the above-mentioned publication, therefore, the vehicle speed is detected when the vehicle has arrived at a steep downhill while traveling in the overdrive range a constant speed, and the overdrive is automatically reset.
That is, it is judged that the vehicle is traveling the downhill at a moment when the speed of the vehicle that is traveling in the overdrive range at a constant speed becomes greater than the target vehicle speed by more than a predetermined value, whereby the overdrive is reset, the transmission is automatically shifted down to produce an increased effect of the engine brake thereby to suppress an increase in the vehicle speed.
According to the device for controlling a vehicle to travel at a constant speed constituted as described above, however, when the vehicle travels a long downhill, the shift-down and the shift-up take place consecutively, which may result in the occurrence of hunting.
That is, when the speed of the vehicle once decreases due to the shift-down while traveling the downhill, it is erroneously judged that the vehicle is now traveling on a flat road, and the automatic transmission is returned (shifted up) to the overdrive position. When the speed of the vehicle increases again, therefore, the transmission is shifted down again to repeat the above-mentioned operation.
According to the conventional device for controlling a vehicle to travel at a constant speed as described above, whether the vehicle is traveling a downhill or has resumed the traveling on a flat road is detected based on the vehicle speed. When the vehicle travels a long downhill, therefore, the shift-down and the shift-up are repeated giving rise to the occurrence of hunting.